This invention relates generally to photolithographic masks and, more particularly, to masks used in processes such as semiconductor device manufacture.
The manufacture of semiconductor devices usually entails several photomasking steps to define and isolate portions of semiconductor surfaces in diffusion, etching and plating processing. Generally, the surface of a semiconductor wafer is covered with photoresist which is in turn covered with a photolithographic mask (hereinafter referred to as a photomask). The combination is then exposed to light and the regions of the photoresist which are exposed by the mask are polymerized. The photomask performs the function of transmitting the light in selected, well-defined areas while blocking light from the remainder of the areas, thus causing a photomask-patterned latent image to be formed in the photoresist layer. During exposure, the photomask is usually located in intimate juxtaposition or even in direct contact with the photoresist-coated surface. Next, the combination is exposed to a reagent which dissolves the non-polymerized photoresist but to which the polymerized photoresist is resistant. The remaining photoresist, comprising an image of the photomask, then serves a purpose; such as, for example, selectively protecting underlying material from attack by liquid or gaseous etchants.
Conventional photomasks used in semiconductor processing are generally formed by selectively locating a photographic emulsion on a glass substrate. A problem encountered with such masks is that the relatively soft emulsion is easily scratched in handling or when using, thus shortening the period of utility of the photomask. Efforts to solve this problem have been made in the past. For example, masks were fabricated by fusing hard, glassy masking material to a glass substrate, see U.S. Pat. Nos. 3,743,417 and 3,816,223. Some improvement was realized, but the opaque mask material was still prominently positioned and thus susceptible to injury. Further improvement was therefore desired.
An object of this invention, therefore, is to provide a photomask that is more resistant to damage than those photomasks heretofore known and to provide a method to make the subject mask.